This invention relates to a polishing pad for use in precision polishing of semiconductor wafers, liquid-crystal glass, hard disks, etc. More particularly, the invention relates to polishing pads for use in chemical mechanical polishing.
As today""s integrated semiconductor circuits have higher packing density and smaller feature size, their fabrication process has become increasingly complicated and the surfaces of semiconductor devices are not always planar. Steps on device surfaces will make conductor paths discontinuous at the steps and increase resistance in limited areas, which in turn cause current discontinuity and reduced interconnect capacitance. In addition, insulation films will have lower ability to withstand voltage, and current leakage can occur.
This is probably the reason why planarization technology has become essential in the process of semiconductor fabrication. One of the promising methods for planarizing semiconductor surfaces is a chemical mechanical polishing technique. Chemical mechanical polishing (hereunder abbreviated as CMP) has evolved from the technology of mirror polishing silicon wafers, and an apparatus for implementing this method is shown in FIG. 1.
A conventional polishing apparatus generally indicated by 1 in FIG. 1 comprises a platen 2 that is driven to rotate and a polishing pad 3 placed on it. A wafer 4 retained by a polishing head 7 is placed in contact with the polishing pad 3. With this setup, the platen 2 is driven to rotate with a downward load on the polishing head 7 so that it oscillates in the radial direction of the platen 2.
Parallel with this movement, a polishing slurry 6 is delivered from a supply nozzle 5 onto the polishing pad 3 so that the slurry 6 is supplied to the underside of the wafer 4 to planarize its outermost surface. To be more specific, the slurry 6 spreads over the polishing pad 3 and as the latter moves relative to the wafer 4, the slurry 6 gets into the gap between the sliding surfaces, thereby polishing the surface of the wafer 4. The mechanical polishing by the relative motions of the pad 3 and the wafer 4 combines synergistically with the chemical action of the slurry 6 to achieve effective polishing.
The polishing pad 3 has in most cases been a sheet of polyurethane foam. However, polishing wafers on a pad in sheet form made of polyurethane foam has involved the following problems.
(A) Since the pad has a dual structure consisting of a sponge layer and an abrasive layer, moisture intrudes from the boundary and the pad swells on the perimeter, leading to increased deterioration in polishing uniformity on the circular edge of the wafer. This results in lower device yield, particularly in the recent years when more than one kind of device is formed on a single wafer.
(B) On account of the foamed structure in the pad surface, compressive deformation tends to occur in surface cells under load and the state of polishing differs from wafer to wafer.
(c) The polishing slurry and debris get into cells in the foamed surface and adhere, often clogging the pad surface. Hence, the polishing performance of the pad decreases and scratches will occur to lower the device yield.
To cope with the problems (B) and (C), the surface of the pad used several times has to be scraped by a suitable device such as a diamond grinder. This dressing step has been an obstacle to the effort of improving process efficiency.
An object, therefore, of the present invention is to provide a polishing pad that has sufficient wet strength to prevent nonuniformity in polishing on the circular edge and which also has resistance to chemicals such as alkalis and acids.
Another object of the invention is to provide a polishing pad that can offer sufficient surface strength to achieve the intended polishing by selecting a suitable kind of abrasive fine inorganic powder and adjusting its loading and which still has little need for dressing.
A further object of the invention is to provide a polishing pad that is functional with a chemical fluid or water in the absence of any polishing slurry or using a polishing slurry loaded with a very small amount of polishing agent.
These objects of the invention can be attained by dispersing an abrasive fine inorganic powder in silicone rubber.
According to the invention, the abrasive fine inorganic powder loaded in a pad substrate not only confers polishing performance but also renders the pad substrate to have a suitable degree of wearability. Hence, as more wafers are polished, the surface of the pad is scraped little by little to expose a new polishing surface.
In essence, the polishing pad of the invention has an abrasive fine inorganic powder dispersed in silicone rubber and this ensures that no fine abrasive powder need be added or only a very small amount of fine abrasive powder need be added during polishing. In addition, the pad surface is scraped little by little as polishing progresses, so there is no need for the dressing operation. In other words, because the fine inorganic powder is dispersed throughout the silicone rubber of the polishing pad, a new surface with inorganic particles is continuously formed as the polishine progresses.
It should be emphasized that there has not been known any idea of polishing pads that need little or no addition of an abrasive fine powder during polishing or those which are scraped little by little on the surface as the polishing process progresses. In addition, no commercial products of such polishing pads have been available to date.